The issue of long-distance data transmission from or to a mobile terminal is of particular concern with regard to connections between computers (machine to machine). This field of transmission is therefore characterized by a need for significantly lower data throughput rates than for image or Internet types of connections.
A first approach to this problem is known, followed by existing data transmission systems such as Orbcomm and Argos, which use constellations of low-orbit satellites (LEO, for Low Earth Orbit). In this approach, the normal mode of operation for each LEO low-orbit satellite requires it to be simultaneously visible firstly by a ground control and connection station and secondly by a user terminal.
The satellite thus serves as a communications link between the two parts, and the latency between acknowledgments of receipt and messages is a function of the distance between the satellite and the ground station (GES, for Gateway Earth Station).
However, the coverage provided by the ground station network of systems using low-orbit satellites, such as Orbcomm and Argos, is limited by the deployment of ground stations (GES) and the existing systems only provide a limited coverage of the Earth in this mode. In effect, each ground station allows coverage over a radius of about 3000 km, and each of these systems comprises some twenty ground stations.
It can therefore easily be seen that the coverage areas have large “blind” areas where the system cannot be used. In particular, these areas cover a large portion of the oceanic areas, even a significant portion of continental areas such as Africa and Australia.
In cases where the LEO satellite has no simultaneous visibility of both the user terminal and the GES ground control station, a type of communications method that stores then sends (known to the person skilled in the art as Store & Forward) must be used. In this method the message is stored on board the satellite, which continues moving in its orbit until it flies over the GES ground station, to which it delivers the stored message.
With this communications method communication delays are long and make two-way communications in acceptable conditions difficult, given that the delays are typically between several minutes and the 100 to 150 minutes duration of a complete orbit of the LEO satellite.
Hybrid telecommunications systems for transmitting data between users are also known. These hybrid systems comprise geostationary satellites and a low-orbit satellite constellation.
In particular a first patent document, FR 2764755/U.S. Pat. No. 6,208,625: Method and apparatus for increasing call-handling capacity using a multi-tier satellite network, can be cited.
This document describes a network formed of LEO and geostationary (GEO) satellites able to communicate to each other. On the ground, user terminals are capable of reception/transmission (Rx/Tx) with LEO and GEO satellites. The LEO component filters the traffic received from the terminals and, depending on the urgent nature of the traffic received, directs this traffic either internally to the LEO or to the GEO.
A second patent document, EP 0883252/U.S. Pat. No. 6,339,707: Method and system for providing wideband communications to mobile users in a satellite-based network, proposes a satellite communications system that permits global coverage, reduced transmission delays (Tx), and maximized use of the system's capacity (wideband communication satellite via the interconnection of several medium-orbit—MEO—and geostationary—GEO—constellations).
The MEO and GEO satellites communicate directly to each other by inter-satellite links, which enables traffic routing (for voice and data) on board the satellites according to certain rules.
In addition, this document proposes having the very high frequency spectrum shared and re-used between the GEO and MEO satellites (e.g. between 40 and 60 GHz), in order to enable the function known under the name “seamless handover” for portable terminals (passing from a mobile network to a fixed network with no interruption to the communication in progress).
It is clear that the current hybrid systems are highly complex, synonymous with high implementation and use costs.